first,i teared it apart about 500 parts in maya,and then import the sd file into the rf and make them into rbd. the rf4 assigns each of the parts different mass randomly,but the value of mass are quite different,eg: some for 0.1 and the other for 100.

here is the question how could i make the mass not so big different? eg: one is 5, and the second is 7,and the third is 10 etc.

I read your post a second time. RealFlow assigns mass based on the volume of the bounding box for each piece. So if their mass is really small its probably a really small piece. Maybe that helps more? Cheers!